Fogged Out. If your boat's clear curtains are scratched or cloudy, remove them for the initial viewing.
Bottom Job. Is your boat's bottom painted? A fresh coat will make it look sharp. Also, use spray paint to touch up your outboard or stern drive skegs where the paint has worn off.
Time to Test. Remove the canvas and stow everything ashore but the safety gear -- you wouldn't want to get caught short if the Coasties stopped you for a spot check. Go light on the fuel and water, too. Your boat will plane easier, handle more nimbly, and hit a faster top end. Limit the sea trial to the buyer only, for the same reasons. Also, having all his friends and family aboard will make the boat feel less roomy.
Bound for Glory. Have all title, registration, extended warranty, and, if available, service records on hand in a binder. It not only looks impressive but also makes you look organized and professional.